<!DOCTYPE HTML>
<html>
<head>
  <script>
    function tweak() {
      // div with style "width:300px; height:100px; background-color:green"
      var shadowDiv = document.createElement("div");
      shadowDiv.style.width = "300px";
      shadowDiv.style.height = "100px";
      shadowDiv.style.backgroundColor = "green";

      var shadowRoot = document.getElementById('outer').attachShadow({mode: 'open'});
      shadowRoot.appendChild(shadowDiv);
    }
  </script>
</head>
<body onload="tweak()">
<div id="outer">
  <div style="width:300px; height:100px; background-color:red;"></div>
</div>
</body>
</html>
